Mass spectrometers have been widely used for analyzing organic compounds. In recent years, analysis of compounds having molecular weights in the order of several thousands have been attempted by the use of the mass spectrometer. For the analyses in such high mass range, it is essential that the mass spectometer has sufficiently high sensitivity and resolution.
Ordinarily, the resolving power R of a magnetic sector type mass spectrometer is expressed as follows: ##EQU1## where S and d represent width of slits for an ion source and a detector, r.sub.m represents a radius of curvature of ion orbit in the magnetic field, .gamma. represents a mass dispersion coefficient, X represents image magnification rate, and .DELTA. represents image expansion due to aberrations. It is apparent from the Eqn. (1) that high resolution can be obtained when the numerator is large and the denominator is small. However, if it is attempted to reduce S for reducing the denominator, the amount of ions capable of drawing out of the ion source is reduced causing a reduction in the sensitivity. For this reason, a high resolution ion optical system can be realized by two methods, one increasing the mass dispersion coefficient .gamma., and the other reducing the image magnification rate X. In either case, the aberrations must be of course reduced, and an efficient detection can be realized by selecting the slit width d to be equal to X.multidot.S+.DELTA..
As for the first method, a mass spectrometer having a maximum resolution of 1 million has been produced by combining a uniform magnetic field and a nonconverging magnetic field. This kind of mass spectrometer, however, cannot have a high scanning speed because the two kinds of magnetic fields must be scanned correlatively. For this reason, this kind of mass spectrometer is adapted only for special use, and it can be concluded that a mass spectrometer utilizing a single uniform magnetic field is far advantageous for the practical use which needs a high scanning speed over a wide mass range. In an optical system utilizing a single uniform magnetic field, however, the value of .gamma. cannot be much increased, ordinarily being restricted in a range of approximately from 0.5 to 1.0.
From the viewpoint of the above described, a virtual image type double focusing mass spectrometer wherein the image magnification rate X can be reduced by the use of a diverging electrostatic field has been worked out, and used practically. In such a kind of mass spectrometer, a virtual image of the ion source slit is formed by the diverging electrostatic field acting as a concave lens, and ions seemingly emitted from the virtual image are introduced into the uniform magnetic field. By forming the virtual image, the image magnification X can be reduced approximately to 1/4, and the resolution can be improved corresponding thereto.
However, it is not practical to reduce the image magnification X smaller than 1/4 by merely enforcing the concave lens action of the diverging electrostatic field in order to improve the resolution because the aberrations abruptly increase with the intensity of the concave lens actions. Therefore, the above described value of the image magnification X is considered to be a lower limit. Several reasons can be given for the increase in the aberrations. The most significant is the effect of the exit boundary of the electrostatic field. More specifically, ions introduced into the diverging electrostatic field are expanded under the concave lens action of the field in the direction of the radius of curvature r, and the degree of expansion increases in accordance with increase in the concave lens action of the field. On the other hand, the disturbance of the field at the exit boundary of the electrostatic field increases with the distance from the central orbit of ions in a direction perpendicular to the electrodes for producing the electrostatic field, that is, in the direction of the radius of curvature r. Accordingly, when it is desired to reduce the image magnification X by increasing the concave lens action, the expansion of the ion beam in the direction of the radius of curvature r increases, thus causing an abrupt increase of the aberrations by the disturbance in the exit boundary of the electrostatic field.